The present invention relates to an electrohydraulic actuator that is particularly adapted to aviation use. It is thus suitable for use in aircraft to control flight control surfaces or variable pitch fan blades, for example.
Electrohydraulic actuators (EHA) are now well known. As shown in the functional diagram of FIG. 5, they essentially comprise a hydraulic actuator 1 for driving a controlled member 2, where the displacement of said member is a function of the speed and the direction of rotation of an electric motor 3 controlling the feed of oil to the actuator by means of a hydraulic pump 4 which is immersed in oil. A hydraulic accumulator 5 compensates for any variations in oil volume that result from the compressibility or the expansion of the oil, in particular under the effect of temperature. The electrical controls 6 and the supply of electrical power 7 are delivered to the actuator via an electronic control unit 8 which acts directly on the electric motor as a function of information concerning the position of the actuator as supplied by a sensor 9.
Structurally, such actuators are made up of a plurality of separate elements that are fitted to the sides of the actuator cylinder and that are connected by means of pipes that are usually flexible, but sometimes rigid, with the exception nevertheless of the motor and pump assembly which is generally integrated in a single housing. U.S. Pat. No. 4,529,362 is a good illustration of such a structure with an external hydraulic tank and actuator. Another embodiment of such an electrohydraulic actuator is disclosed in French patent No. 2 489 897 which shows a relatively compact hydraulic assembly comprising two adjacent portions, one comprising the actuator and its hydraulic directional control valve, the other combining the motor, the pump, and the accumulator. Although that better-integrated structure gives satisfaction overall in most industrial applications, in the context of aviation use, e.g. use requiring the assembly to be set into rotation or else requiring small radial extent, it presents drawbacks associated with its asymmetrical configuration (unbalanced configuration) that gives rise to high levels of vibratory stress.
The present invention proposes to mitigate the drawbacks of prior art devices by means of a self-contained electrohydraulic actuator that is axially symmetrical and radially compact. A particular object of the invention is to propose an actuator that is balanced, so that said actuator can be set into rotation, for example in order to make it fit for certain particular uses in aviation.
These objects are achieved by an electrohydraulic actuator comprising: a hydraulic actuator comprising a piston secured to a member to be controlled and capable of moving in a cylinder under drive from a hydraulic fluid; a hydraulic pump feeding both faces of said piston with hydraulic fluid via channels integrated in the walls of said cylinder; an electric motor immersed in the hydraulic fluid, and controlled and powered electrically by an electronic control unit, driving said hydraulic pump in rotation; and a hydraulic accumulator for compensating variations in the volume of the hydraulic fluid; wherein said hydraulic actuator, said hydraulic pump, said electric motor, and said hydraulic accumulator have a common axis of symmetry and are mounted in alignment in a common housing that is leaktight to the hydraulic fluid, thereby forming an axially symmetrical self-contained hydraulic assembly.
This particular arrangement leads to an actuator structure that is highly simplified, fully integrated and thus particularly compact, and easy to implement and to replace because it is hydraulically self-contained. Risks of leakage are also greatly reduced because of the absence of any rotary couplings or pipework. Above all, the axial, in-line structure of this actuator makes it possible to eliminate all of the unbalance that is to be found in prior art devices and that is prejudicial to vibration-free operation should the actuator need to be set into rotation.
Preferably, the hydraulic pump is a swash-plate pump with axial pistons. The pistons move in a cylinder system which is rotatable about said common axis of symmetry, and said swash-plate is stationary and secured to said housing. This configuration of a rotary cylinder system makes it possible to avoid using check valves and thus to reduce simultaneously the number of parts, the weight, the size, and the risk of the actuator breaking down.
Said electric motor is preferably of the brushless type, having permanent magnets or being an asynchronous motor.
In an advantageous embodiment, said cylinder system of the hydraulic pump is integrated with said rotor of the electric motor. This configuration of the motor and pump assembly thus forms an integrated electrical pump.
Preferably, the hydraulic accumulator comprises a metal bellows assisted by a spring. It may be disposed axially beneath said pump body or in said actuator cylinder. Advantageously, the actuator of the invention further comprises balancing means for eliminating the unbalance caused by the swash-plate when the actuator is set into rotation.